1. Field of the Invention
This invention relates to speed indicators and more particularly to a device for measuring the rate of travel of a marine craft through water. The device is especially adapted for measuring the rate of speed of a sail boat in water.
2. Description of the Prior Art
Devices for measuring the speed of marine craft have been in existence for a long time. In U.S. Pat. No. 168,432, issued to S. D. Trenchard in Oct., 1875, a device is described in which a spring encased in a cylindrical structure is attached to a drag which is pulled through the water. The drag resists being pulled through the water and thereby compresses the spring which is reflected on the scale alongside the long axis of the cylindrical structure. During compression, the spring will rub along the inside of the cylinder. This friction will affect the accuracy of the readings. The device is attached to a boat railing or other solid structure on the boat in a way which allows it to pivot in a holder in order to maintain the alignment of the two with the drag behind it. The device can only rotate in two directions and thereby it is not entirely free to align the tube with the drag to obtain the maximum responsiveness.
Another device was patented in U.S. Pat. No. 290,840, to B. E. Blakeslee, in Dec. 1883. That device was fixed to the railing of the rear of a boat and, because of the way it is anchored to the railing, there is an angle on the line holding the drag and the indicating device such that the lines of force are not aligned with the instrument itself to obtain the most efficient, accurate reading.
In U.S. Pat. No. 2,081,557, issued to R. H. Pierce, et al, in May, 1937, a rather complex device is shown which suffers in addition to its complexity from the same defect as the Trenchard patent in that it is attached to the ship by a hinged swivel which only allows rotation in two directions and thereby decreases the opportunity for the drag and the instrument to maintain its alignment.
There are a number of devices which act to measure the angle at which a weight will rest at equilibrium when it is being pulled through water by the boat. Thus, the amount of deviation from the vertical as reflected by the deflection of a line carrying a weight can be an indication of the speed of the boat. The operating principle of these devices is different than the ones previously mentioned and the one described by the applicant herein. These devices are claimed in U.S. Pat. No. 2,707,878, to W. E. Cameron, et al, in May, 1955, U.S. Pat. No. 2,152,768, to D. M. Myers, in April, 1939, and U.S. Pat. No. 3,089,335, to E. A. Ebert, in May, 1963.
A number of other patents also depend on trolling a device for the measurement of speed of a boat. However, in each of these devices friction points are present because the line from the drag to the instrument is not a straight line due to its manipulation over or around bends. This deflection from the straight-line path between the drag and the indicating device leads to error because of friction generated at each point. Patents embodying this problem are U.S. Pat. No. 477,284, issued to J. J. Townsend, in June, 1892, U.S. Pat. No. 3,706,223, issued to H. R. Dickens, in December, 1972, and U.S. Pat. No. 3,901,076, issued to R. E. Grayson, in August, 1975.
Other complex devices are known which require through-the-hull fitting and thereby greatly increase the overall cost of such devices.
Mainly, the prior art devices are deficient because the line from the drag passes over points of friction before the force pulling on the line is measured which interferes with the accuracy of readings. Pulleys and other devices can be incorporated into a device but at the expense of making it more complex and expensive.
The prior art devices also fail to disclose a stable design for the drag.